Through-the-Wall Imaging
Through-the-wall-imaging (TWI) can be used to detect objects inside an enclosed structure from the outside. In TWI, a transmitter emits an electromagnetic (EM) radar pulse, which propagates through a wall. The pulse is reflected by the objects on the other side of the wall, and then propagates back to a receiver as an impulse response convolved with the emitted pulse. Typically, the transmitter and receiver use an antenna array.
Depending on a dielectric permittivity and permeability of the wall, the received signal is often corrupted with indirect secondary reflections from the all, which result in ghost artifacts in an image that appear as noise. Wall clutter reduction techniques attempt to eliminate the artifacts that arise from the multi-path reflections TWI.
Compressive Sensing
Compressive sensing (CS) and other sub-Nyquist sampling and acquisition methods can be used by sparse, undersampled radar array systems. The antenna array enables radar signal acquisition and imaging using significantly fewer array elements compared to conventional array structures, thus significantly reducing the array implementation cost.
Sparse arrays have an average inter-element spacing much larger than half the wavelength of the transmitted signal, which is the Nyquist interval for may processing. This is achieved using non-uniform element spacing, which eliminates fundamentally unresolvable ambiguities known as grating lobes.
While conventional methods have been used to recover the acquired image, those methods suffer from the increased sidelobes exhibited by those arrays. However, sparse recovery methods are robust to sidelobes, thus enabling imaging using significantly fewer array elements. As used herein, “sparsity” is not a relative term, but rather a term of art used to refer to data with mostly zero values, and only a few non-zero values.
In U.S. application Ser. No. 13/947,426, “Method and System for Through-the-Wall imaging using Sparse inversion for Blind Multi-Path Elimination,” filed by Mansour, on Jul. 22, 2013, targets are detected in a scene behind a wall by transmitting a pulse through the wall. A primary impulse response is detected by a sparse regularized least squares inversion applied to received signals corresponding to the reflected pulse. A delay operator that matches the primary impulse response to similar impulse responses in the received, signals is also determined. A distortion of the pulse after the pulse passes through the wall hut before the pulse is reflected by the target can also be determined. The distortion is used in an iterative process to refine the detection of the target and to suppress ghosting artifacts.